The present invention relates to a method for forming a thin silicon layer on an insulator, particularly to such a method which introduces a boron doped etch stop for controlling removal of the silicon, and more particularly to such a method, using a short wavelength laser for performing laser assisted doping and activation, and which includes reducing the remaining surface roughness of the silicon thin film and/or etch back during the fabrication of the thin film of silicon on glass.
There has long been a desire to perform single-crystal silicon (SCS) processing with glass substrates to achieve single-crystal silicon (SCS) transistors on glass or other insulators.
Silicon-on-insulator (SOI) technologies have advanced dramatically in recent years toward the goal of producing thin silicon films on insulating substrates, such as glass or oxidized silicon. Components, such as transistors, fabricated in SOI films have the potential for increased mobility, reduced parasitic capacitance and leakage current as well as improved radiation hardness due to reduced junction sidewall area and elimination of bottom junction area. To date, there has been no success in achieving single-crystal silicon device fabrication on less expensive glass substrates incapable of withstanding temperatures more than 600.degree. C. Others have achieved this with expensive glasses, such as Corning 1729 using 800.degree. C. and Corning 1733 at 600.degree. C. with compromises. SOI transistors on glass substrates are particularly attractive for sensors and displays, although many other applications are possible, such as actuators, high temperature electronics, optoelectronics, and radiation hard electrons.
A wide variety of techniques have been proposed for realizing thin amorphous silicon and polysilicon films compatible with high-performance devices on an insulating substrate. One prior approach involved crystallization of deposited polysilicon films using thermal energy derived from incident beams. See R. A. Lemons et al., "Laser Crystallization of Si Films on Glass", Appl. Phys. Lett., Vol. 40, pp. 469-471, 1982. This approach has met with some success, but device performance is still limited by problems associated with silicon crystal quality. Device properties in polysilicon or in amorphous silicon films are generally still less favorable. Another prior approach involved a process for producing silicon films on oxidized silicon and involves bonding two oxidized wafers together at high temperature and then thinning one wafer to produce a silicon-on-oxidized-silicon film. See J. B. Laskey et al., "Silicon-on-Insulator (SOl) by Bonding and Etch-back", Dig. 1985 IEEE Int. Electron Devices Mtng., December 1985, pp. 684-687. However, this process does not use a glass substrate which would make it useful for sensor/display applications, and voids during the bonding process are a concern. A more recent approach involved a process using electrostatic bonding of a silicon wafer to expensive glass, such as Corning 1729 glass, and the subsequent thinning of the wafer using doping-sensitive etchants to retain only the epitaxial layer. See L. J. Spangler, "A Technology for High-Performance Single-Crystal Silicon-on-Insulator Transistors", IEEE Electron Device Letters, Vol. EDL-8, No. 4, April 1987, pp. 137-139. This process was effective for fabricating high-performance transistors in single-crystal silicon on glass substrates and is potentially compatible with both bipolar and MOS structures. However, this process used an alkaline-earth aluminosilicate high-temperature glass substrate having an anneal point of 853.degree. C. as the substrate, whereby relatively standard integrated circuit (IC) process involving a temperature of about 800.degree. C. could be used to fabricate the transistors, without damage to the glass substrate. In addition, this process involved electrostatic bonding of the silicon wafer to the glass substrate, requiring application of a typical voltage of 900 V for about thirty (30) minutes and resulted in bonding voids due to surface irregularities. Furthermore, Corning no longer commercially manufactures glasses capable of withstanding temperatures in excess of 600.degree. C.
While these prior approaches advanced the SOI technologies, there has been a need in this art for a process for forming thin film silicon electronic devices on less expensive glass substrates. One approach to achieving the thin silicon films necessary to achieve this approach is exemplified by U.S. Pat. No. 5,013,681 issued May 7, 1991 to D. J. Godbey et al. A solution to the above-mentioned problems is provided by the present invention which involves the use of a laser to irradiate the surface of the thin silicon layer in the presence of a suitable doping gas, of sufficient energy to melt the silicon surface causing gas adsorption and dissociation on the surface of, and dopant incorporation through liquid phase diffusion in, the melt. Thus, this prior need in the art is satisfied by the present invention whereby a thin film of silicon is fabricated on a glass substrate and subsequently doped with electrically active species without damage to the glass. The method involves the use of a glass substrate as the handle wafer for a thin single-crystal device quality silicon layer, with the silicon substrate prepared with etch stop layers and an epitaxial thin film of device quality single-crystal silicon, with the two substrates being bonded anodically, whereafter the silicon substrate is etched away and the extra layers are removed, leaving a thin film of device quality single-crystal silicon on the glass substrate. The method may additionally involve using an excimer laser for forming one or more boron doped etch stops and reducing the surface roughness of the thin films in combination with the etching procedures.